Generally, based on the on-state orientation scheme of liquid crystal major axis, the direction of the axis is referred to as directors, there are two primary types of liquid crystal displays LCDs. The first type of LCD has directors that rotate mainly in a plane orthogonal to a substrate and the second type has directors rotating mainly in a plane parallel to the substrate.
The most typical mode of the first category is the twisted nematic liquid crystal display TN LCDs, in which the electrodes are configured on both of the two substrates. The TN LCD is usually arranged to be twisted at an angle of 90 degrees and shows high transmittance, low power consumption, and easy fabrication. However, the TN LCD exhibits a narrow viewing angle because its liquid crystal directors on the two substrate surfaces are perpendicularly aligned to each other.
Various methods have been disclosed to solve the problem of the narrow viewing angle in typical LCD devices. One effective way of solving the problem is the introduction of multi-domain structures in the LCD devices. However, multi-domain in TN typed LCDs such as the two or four-domain TN LCDs are limited in improving the viewing angle to achieve a contrast ratio approximately >10:1 confined in a viewing cone at approximately 35 degrees.
Film compensation is another effective method to enhance the viewing angle of TN-LCDs. The major breakthrough was made by Fuji Photo Film Co., which used discotic liquid crystals as a wide viewing film to compensate the dark state of TN LCDs as described in Mori et al, Late-News Paper: Novel Optical Compensation Film for AMLCDs Using Discotic Compound, SID (1997), pp. 941-944. The viewing angles of TN LCD with wide viewing film could be expanded the 10:1 contrast ratio cone to approximately 60 degrees. However, a 60-degree viewing angle is inadequate for large LCD TVs and monitors, especially for solving the problem of non-uniform gray scale at larger viewing angle.
Other display modes for the twisted nematic LCDs, in which the liquid crystal directors mainly rotate in the plane perpendicular to the substrates, for solving the wide-viewing angle problem include the film-compensated multi-domain vertical alignment described in Takeda et al, A Super-High Image Quality Multi-Domain Vertical Alignment LCD by New Rubbing-Less Technology, SID (1998) p. 1077, and patterned vertical alignment mode described in Kim et al, Domain Divided Vertical Alignment Mode with Optimized Fringe Field Effect, ASID (1998) p. 383 liquid crystal display mode. Each has advantages and disadvantages, however problems still exist. Usually negative liquid crystal materials are preferred in the modes just described and they require complex fabrication methods such as protrusion in multi-domain vertical alignment.
Besides the above-mentioned approaches, another effective solution for wide viewing angle problem is to replace TN LCDs with in-plane switching mode LCD devices as described in U.S. Pat. No. 5,598,285 issued to Kondo et al on Jan. 28, 1997. The in-plane switching typed LCDs belong to the second category in which the electrodes are configured on the same substrate and LC directors mainly twist in a horizontal plane as shown in FIG. 1a. The viewing angle characteristic is improved as the liquid crystal directors in the voltage-off state are horizontally aligned, or aligned in parallel to the substrates. With this liquid crystal director distribution, the viewing angle is less azimuthally dependent and wider and can attain approximately a 10:1 contrast ratio at approximately 60 degrees without use of compensation film and the viewing angle is more symmetrical than that of the TN LCDs. Therefore, the in-plane switching mode LCD is more popular than the twisted nematic LCD.
However, in-plane switching mode LCDs also have limitations. FIG. 1a is a schematic of the structure of a prior art in-plane switching LCD having a first and second substrate 101a and 101b, respectively and a liquid crystal layer 104 disposed between the first and second substrates 101a and 101b. The common electrode 102 and pixel electrode 103 in the in-plane switching LCD are often made of opaque metals such as aluminum which blocks the light above the pixel electrode 102 and common electrode 103 regions. FIG. 1a also shows the corresponding transmittance curve versus position in the prior art IPS LCDs with opaque electrodes. As shown, there are two dark zones where the transmittance is approximately zero above the surface of the pixel and common electrodes 102 and 103, respectively. As a result, the aperture ratio and the transmittance in EPS mode are usually low when compared to the TN LCDs. In order to improve the transmittance, U.S. Pat. No. 6,924,863 issued to Nishida et al. issued to on Aug. 2, 2005, disclosed common electrodes and the pixel electrodes that are composed of transparent material such as indium tin oxide (ITO).
Therefore, a need exists for a display mode that combines the wide viewing angle of the in-plane switching LCDs with the high transmittance of the twisted nematic LCDs.